Investigation of solid state imidization reactions of the vapour deposited azo-polyimide thin films by FTIR spectroscopy

Journal of Molecular Structure 1074 (2014) 100–106

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Investigation of solid state imidization reactions of the vapour deposited azo-polyimide thin films by FTIR spectroscopy Anton Georgiev a,⇑, Dean Dimov b, Erinche Spassova b, Jacob Assa b, Gencho Danev b a

University of Chemical Technology and Metallurgy, Department of Organic Chemistry, 8 St. ‘‘Kliment Ohridski’’ Blvd., 1756 Sofia, Bulgaria Institute of Optical Materials and Technologies ‘‘Acad. Jordan Malinovski’’, Department of ‘‘Nanostructured Materials and Technology’’, Blvd. 109 ‘‘Acad. G. Bonchev’’ Boulevard, 1113 Sofia, Bulgaria b

h i g h l i g h t s  Experimental parameters for Azo-PI films preparation by solid state reactions have been established.  The structure of Azo-PI films has been determined by FTIR spectroscopy.  The kinetic of imidization has been investigated by FTIR spectroscopy at different temperatures.  The appropriate temperature for solid state imidization is 300 °C for 1 h.

a r t i c l e

i n f o

Article history: Received 10 December 2013 Received in revised form 26 May 2014 Accepted 27 May 2014 Available online 4 June 2014 Keyword: Azo-polyimide Thin films Solid state reactions Imidization Vapour deposition FTIR spectroscopy

a b s t r a c t In this paper solid state imidization reactions of azo-polyimide thin films prepared at different temperatures have been investigated by FTIR spectroscopy. The starting precursors DAAB (4,40 diaminoazobenzene) and PMDA (pyromellitic dianhydride) were vacuum deposited as thin films. It was estimated that the necessity of the following thermal treatment 1 h at 300 °C are appropriate conditions for imidization of the initial PMDA–DAAB mix to Azo-PI (azo-polyimide) film. The parameters have been selected experimentally (temperatures, deposition rates, mode of substrates, etc.), for the vapour deposition of the precursors in view to prepare layers with a high imidization degree, reproducible composition and structure, which determined the film properties. Due to their magnetic anisotropy and cis–trans–cis isomerization cycle the azo-polymer films are promising material for application in optics, optoelectronics, sensors, etc. For this reason a study of the kinetics of solid state imidization reactions at different temperatures is carried out. It is identified and discussed that the imide III band intensity at 1363–1381 cm1 characterizing imide ring is different at three temperatures. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Over the past two decades, research interest has been focused on polymer films containing azobenzene fragments. These films are of interest because of their potential application as photoactive materials for optical recording, biosensors, reversible photoswitch, liquid crystal polymer, photosensitive elements, optical storage information, holographic effect, etc. It is known that polyimides containing azobenzene fragments ‘‘main chain’’ type have very good chemical and thermal stability and mechanical properties, which makes them preferable to other polymers containing azogroup [1–3]. It is known that photoinduced isomerization between Trans and Cis isomers (Fig. 1) and the occurrence of magnetic ⇑ Corresponding author. Tel.: +359 887 94 06 83. E-mail address: [email protected] (A. Georgiev). http://dx.doi.org/10.1016/j.molstruc.2014.05.070 0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

anisotropy are the main properties of these compounds, which determine their practical application [4,5]. Trans–Cis–Trans isomerization cycle leads to shift of kmax in UV–VIS absorption spectrum and isomers have different properties [5–7]. The application and properties of Azo-PI (azo-polyimides) based on the possibility of nanosized film formation. The literature describes many experimental data related to the application of thin films, where the attention is mainly focused on photoorientation and optical properties [5,8–10]. It is known the formation of thin Azo-PI layers by spin coating via PAA (poly (amic) acid) [11,12], or by vacuum deposition of precursors like DAAB (4,40 -diaminoazobenzene) and PMDA (pyromellitic dianhydride) [13–15]. The quality and structure of the prepared films are highly dependent on the preparation method. The main advantage of vacuum deposited thin films over spin coating is the absence of solvents and eliminating the need for

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A. Georgiev et al. / Journal of Molecular Structure 1074 (2014) 100–106 R

R

:

E = h.υ N N

:

:

N

.

E1 = h.υ

R

E = h.υ

.

N

:

E1 = h.υ

: :

N N

E > E1

E > E1

R

R

Trans (E) - isomer

R

[Transition state]

Cis (Z) – isomer

Fig. 1. Photoisomerization of azo-polyimide.

removal. Both methods have advantages and disadvantages. One of the advantages of vacuum technology is the possibility to fabricate films with a controlled, uniform and reproducible growth, structure and thickness. A disadvantage may be noted that compounds with high molecular weight evaporate difficultly or in some cases this process is impossible [13–15]. However, vapour deposition of organic nanosized layers is significantly greater prospect for development of nanotechnology. Deposition of precursors under high vacuum and subsequent thermal treatment of the film is related to running of imidization reactions in the solid state. The method is compatible with the technologies in optics, electronics, which require high purity, low moisture and allows to films formation with uniform density, where the probability of defects formation is smaller than spin coating [16,17]. Simultaneously, the FTIR spectroscopy is an appropriate tool for investigating the kinetics of solid state reactions [18]. From a practical point of view, it is fast, accurate and easily reproduced method. The shape and type of spectral bands depend on the purity and physicochemical properties of the polymer (conformation, crystal structure, etc.). The data obtained for structural-group composition is used for qualitative and quantitative evaluation of thin layers [19,20]. Our previous investigations were focused on the preparation of polyimide and nanocomposite films by vacuum technology and solid state imidization [15–17,21]. In this paper, we discuss the parameters for the synthesis of Azo-PI films and kinetics of imidization at different temperatures. Our studies were focused on Azo-PI film obtaining by vapour deposition of the precursors DAAB and PMDA and subsequent solid state imidization. The aim was to: (i) optimize the preparation of azo-polyimde films by vapour deposition through a selection of precursors, deposition conditions, defined composition and structure and solid state imidization reactions; (ii) study the influence of temperature on imidization for obtaining azo-polyimides films with reproducible composition, which is related to the reproducibility of their optical, electrical and magnetic properties.

Both compounds were purchased with analytical protocols and quality certifications according to EU standards.

Preparation of vapour deposited azo-polyimide layers The experiments were carried out with vacuum deposited films prepared on soda–lime–glass plates (Fig. 2). Only for FTIR spectroscopy, as substrates we have used the transparent for IR irradiation KBr tablet substrates. The vacuum chamber was evacuated by oil diffusion pump to a pressure of <7  104 Pa. The used precursors PMDA and DAAB were evaporated simultaneously from two independent thermally-heated Knudsen type sources at deposition rates from 0.2 to 2 A/s. The final thickness about 250 nm, and the deposition rates were strictly controlled by quartz oscillators on linearly moving substrates by speed 1 mm/s to evaporation flow. The typical evaporation temperatures were 240–260 °C for DAAB and 120–145 °C for PMDA. Thus, the molar ratio in the precursor flux of 1:1 was ensured.

Experimental Used materials The precursors for studying films were purchased as follows: (i) PMDA (pyromellitic dianhydride) was supplied by Merck with a purity of 99% as a white crystalline substance with MP 283– 286 °C; (ii) DAAB (4,40 -diaminoazobenzene) supplied by Enamine with a purity of 98.9% as yellow–orange crystalline solid with MP 238–241 °C;

Fig. 2. Scheme of the system for preparation of vapour deposited azo-polyimide films:(1), (2) sources; (3) quartz oscillators; (4) main vapour streams from PMDA and DAAB; (5) vacuum valve; (6) moving band with substrates.

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The obtained films were placed successively in a closed chamber for thermal treatment and imidization at different temperatures (200, 250 and 300 °C) for 1 h. The camera was equipped with an infrared heater and the temperature increased approximately 10 °C per 1 min. The chosen of the temperature interval is based on the rising unstability of the initial azo-compound with the increasing temperature at higher then reaction conditions levels. DTA and TGA analysis The parameters of evaporation temperature of the precursors, temperature of solid state imidization were selected based to the DTA (Differential Thermal Analysis) and TGA (Thermo Gravimetric Analysis). For DTA and TGA analysis of samples was used annealed to constant weight crucible Al2O3, in atmosphere of inert gas (argon) at heating rate of 10 °C/min. DTA analysis, monitoring the temperature of the sample at uniform heat supply. Measure the difference in temperature of the sample and standard (inert gas) when the sample changes were made related to the thermal effect of chemical reactions, phase transformations and structural changes. TGA consist of measure the mass of the sample as a function of its temperature. The observed change in the mass is related to the purity or composition of the sample. FTIR spectroscopy measurements FTIR spectra of the vapour deposited films on a KBr substrate have been recorded on a Bruker Tensor 27 FTIR spectrophotometer in the range 4400–600 cm1 with resolution 2 cm1. For kinetic estimations of solid state imidization reactions of the thin layers, the spectra have been normalized at 1723 cm1. The band was chosen because of the quantity of ˜C@O groups had not changed during the imidization. Each complex and broad bands were analyzed by Fourier Self Deconvolution. The spectra have been subjected to a line shape analysis by the GRAMS and SpekWin32 software. Measurement of thickness The thickness was measured by profilometer ‘‘Talystep’’ Taylor Hobson. It measures the thickness by traversing the stylus either across a test groove formed in the deposited film. Vertical movement of the stylus is amplified electronically and recorded as a graphical representation of the differences in level between the surfaces of the substrate and the deposit. Results and discussion The development of nanotechnology and polymer chemistry required innovative approaches for obtaining of nanosized films. The use of vacuum technology for the preparation of polymer thin films allows to be studied solid state reactions. Our proposal for obtaining the vapour deposited films of Azo-PI by solid state imidization on substrate surface is based on computerized technology that provides high purity, controlled thickness and uniform film thickness. The vapour deposition of precursors PMDA and DAAB is associated with running of solid state reaction to PAA on the substrate surface. In the course of this stage of reaction temperature required higher than 90 °C. Scheme 1 represents the reaction between both precursors. Fig. 3 shows the FTIR spectra of PAA resulting film, which was compared with the spectra of starting films precursors DAAB and PMDA.

In the spectrum of vapour deposited DAAB film is identified typical bands for ANH2 group 3379 and 3346 cm1 corresponding to symmetric and asymmetric stretching vibrations and deformation vibrations 1618 cm1. There are characteristic bands of benzene rings 1592 and 1504 cm1 for CAC stretching vibrations. The anhydride ring of PMDA is identified from the bands 1745 and 1732 cm1 corresponding to symmetric and asymmetric vibrations of ˜C@O. The simultaneous deposition of both precursors on a substrate occurs solid state reaction to PAA (Scheme 1). In the spectrum of Fig. 3 typical bands for PAA in the range 2700– 3400 cm1 are observed associated with a complex band of AOH group with shoulder at 3340 cm1 for ANH stretching vibrations. There are two bands of ˜C@O groups: (i) 1712 cm1 COOH; (ii) amide I band 1652 cm1 ACOANH. Amide II band 1540 cm1 is multiple and wide. The characteristic band of deformation vibrations for the CAOAH is 1401 cm1. A new band appeared at 1364 cm1 which is associated with deformation CAN vibrations (amide III band), where in the spectrum of DAAB and PMDA is minimum observed. Compared with classical methods for films obtaining (spin coating), here solvents and two or three step synthesis were not used and the possible side reactions were minimized [22]. It is known that upon synthesis of azo-polymer films from solution running azo-imine tautomerism (Scheme 2) [23]. These are negative forms for imidization processes in the polymer thin film formation and their application in high technology. Fig. 3 shows that the individual spectrum of DAAB film the band for imine form is missing, which is identified in the range 1640–1690 cm1 (C@N, stretching vibrations) [24]. Vapour deposited PAA film was subjected to thermal treatments for solid state imidization reactions (polycyclodehydration) to linear Azo-PI (Scheme 3). Fig. 4 presents FTIR spectra of PAA and Azo-PI films obtained after thermal treatment for 1 h at 300 °C. In the spectrum of Azo-PI bands of ˜C@O groups of the imide ring 1779 and 1726 cm1 for asymmetric and symmetric vibrations are clearly identified. The band at 1381 cm1 characterized the imidization process (imide III band), associated with the CANAC stretching vibrations of imide ring. For conformed of solid state imidization reaction can be judged by the minimum of absorbance 1652 cm1 (amide I), 1540 cm1 (amide II) and 1401 cm1 for deformation CAOAH vibrations of the acid compared with PAA spectrum. Azo-PI was obtained by us for the first time by vapour deposition of precursors and next solid state reactions [25]. It is known obtained from solution as thin film by spin coating [5,12,26]. Azo-polymers films are important for optical technologies and nanotechnology due to the magnetic anisotropy and Cis–Trans–Cis isomerization cycle. For this reason it is important to investigat the kinetics of solid state imidization reactions at different temperatures to produce high quality and purity films. In Fig. 5 shows the FTIR spectra Azo-PI films obtained after 1 h thermal treatment at temperatures 200, 250 and 300 °C. The ratio of the imide III bands intensities at 1363–1381 cm1 characterized imide ring is different at three temperatures. When increasing the temperature, place of the bands shifted to higher frequencies, which is associated with the formation of the greater amount of imide rings, respectively, increasing the force constant of CANAC vibrations. In the spectrum of Azo-PI obtained at 200 °C shows that imidization is not fully took place, where the bands identified amid I and amid II at 1653 and 1597 cm1 of the PAA, and deformation vibrations of CAOH 1200–1300 cm1 related to acid are observed. The increasing of temperature with 50 °C, imide III band shifts to higher frequencies by 14 cm1 and the typical bands of PAA missing. In the spectrum of Azo-PI film prepared at 300 °C imide III band at 1381 cm1 is narrow and shifted at higher frequency compared to the films spectra at 200

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O H2N

N N

O

+

NH2

O Vapour deposition

O

O

t > 90 0C

O

DAAB

PMDA O

O HOOC

N H COOH

HOOC H N

N N

COOH

O

O

n

PAA Scheme 1. Reaction between DAAB and PMDA to PAA.

1,1

1540

1592

1364

0,7

Absorbance/a.u.

0,9 1401

0,8

Absorbance/a.u.

3379

0,6

1

0,7

1732

1712 1618

0,5 0,4 0,3 0,2 0,1

0 3600 3500 3400 3300 3200 3100 3000 2900 2800 2700 2600 2500

1397 1652

0,6 1745

0,5

3346 3477

Wavenumbers/cm

-1

1504

0,4 0,3 1693

0,2 0,1 0 1900

1800

1700

1600

1500

1400

1300

1200

1100

1000

900

800

700

Wavenumbers/cm-1 Fig. 3. FTIR spectra of vapour deposited films 250 nm of: ——— PAA; ———— DAAB; ––––PMDA.

a

..

HN H

N N

NH2

H N N

HN

NH2

b

H+ +

H

HN

N HN

NH2

Scheme 2. Azo-imine tautomerism of DAAB.

Poly(amic) Acid (PAA)

t > 180 0C -2nH2O

O *

N N

N O

O N

N N

*

O n

Scheme 3. Polycyclodehidratation reaction (imidization) of PAA to Azo-PI.

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A. Georgiev et al. / Journal of Molecular Structure 1074 (2014) 100–106 1,1 1723

1540

1364

1 0,9 1401

Absorbance/a.u.

0,8 0,7

1381

1712

0,6

1652

0,5 1502

0,4 0,3

1779 1598

0,2 0,1 0 1900

1800

1700

1600

1500

1400

1300

1200

1100

1000

900

800

700

Wavenumbers/cm-1 Fig. 4. FTIR spectra of vacuum deposited films 250 nm of: ——— Azo-PI thermally treated 1 h at 300 °C and ———— PAA.

1,1

1723 1363

1 0,9 1377

0,8

Absorbance/a.u.

1381

0,7 1500

0,6 0,5 0,4

1684 1597 1781

0,3

1653

0,2 0,1 0 1900

1800

1700

1600

1500

1400

1300

1200

1100

1000

900

800

700

Wavenumbers/cm-1

DTA

0,4 0 0,2 0,0

TGA

0,8 0,6 0,4 DTA

0,2 0,0

-2 0

100

200

300

400

Temperature / 0 C

500

0

100

200

300

400

500

c

22 20 18 16 14 12 10 8 6 4 2 0 -2 -4 -6 -8

1,0 TGA

0,8 0,6 0,4 DTA

0,2 0,0 0

Temperature / 0 C Fig. 6. TGA and DTA of: (a) DAAB; (b) PMDA; (c) molar ratio 1:1 PMDA: DAAB.

100

200

300

400

Temperature / 0 C

500

Differential temperature

2 0,6

22 20 18 16 14 12 10 8 6 4 2 0 -2 -4 -6 -8

1,0

weight loss/%

0,8

b weight loss/%

4

TGA

1,0

Differential temperature

weight loss/%

a

Differential temperature

Fig. 5. FTIR spectra of vapour deposited films 250 nm of: ——— Azo-PI thermally treated 1h at 200 °C; ———— Azo-PI thermally treated 1 h at 250 °C; –––– Azo-PI thermally treated 1 h at 300 °C.

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Conclusions

O

O

The preparation of ‘‘main-chain’’ polyimide films containing azo-group by vapour deposition of the precursors is developed. After thermal treatment of deposit layers of the precursors DAAB (4,40 -diaminoazobenzene) and PMDA (pyromellitic dianhydride) have solid state reacted to Azo-PI (azo-polyimide). The kinetic of imidization at different temperatures has been investigated by FTIR spectroscopy. It was estimated that the appropriate temperature for solid state imidization in the investigated interval is 300 °C for 1 h. It was determined that evaporation pressure <7  104 Pa and evaporation temperatures 240–260 °C for DAAB and 120– 145 °C for PMDA are appropriate for Azo-PI (azo-polyimide) film preparation. The molar ration 1:1 of the evaporated precursors ensures high imidization degree.

N

O

N

N

isoimide

imine

Fig. 7. Structure of isoimide and imine.

and 250 °C. In the range of molecular vibrations 1000–1300 cm1 the bands are well shaped for typical polymer spectrum [27]. Aromatic polyimides are well-known and characterized by high thermal and chemical stability, good physicochemical characteristics and good mechanical properties [28–30]. For the determination of the imidization temperature range, we have taken into account the initial results obtained by TGA and DTA analyses (Fig. 6). We previously determined decomposition temperatures and a loss of mass of the starting precursor at the molar ratio PMDA:DAAB = 1:1. It has been found, that temperatures above 300 °C starting changes in the composition of the compounds, where the observed loss of weights is related from the decomposition of the substances. Obtaining the polyimides both by classical methods and vapour deposition, a competitive reaction takes place to isomide and imine (Fig. 7). These forms are also unfavourable for building nanosized films due to defects in the layer structure and changes of its physicochemical properties [30]. Nucleophilic substitution of solid state imidization reactions of Azo-PI run with high regioselectivity and the possibility of the side reactions is minimized. Spectral bands characterizing isoimide and imine are locate in the range 1640–1690 cm1, where in the spectrum of Azo-PI obtained at 200 °C is observed shoulder at 1684 cm1 from imide ring (Fig. 5). Establishing the optimum temperature for the imidization degree is an important aspect of the application of Azo-PI film and therefore the study of kinetic is directly related to the occurrence of magnetic anisotropy and photoisomerization of azo-polymers. At the temperatures 200 and 250 °C (Fig. 5) the imide III bands are broad and shifted to lower frequencies (1363 and 1377 cm1) compared to the film spectrum at 300 °C (1381 cm1). These results are due to incomplete imidization and formation of isoimide, which at 300 °C converted to imide by the intramolecular rearrangement according to Scheme 4 [31,32]. From the spectra in Fig. 5, is obvious that the imide III band is located at 1381 cm1 and characterized imide ring completely. The bands shape for ˜C@O imide groups (1779 b 1726 cm1) is clear, the range of molecular vibrations is well defined for typical polymer spectrum. From the experimental results, it can be accepted that for the studied temperature interval the experimental conditions are appropriate at optimum temperature for imidization of Azo-PI 300 °C at 1 h.

O O

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O 0

t > 200 C

O

..

N

N

t > 200 0C

..O Scheme 4. Intramolecular rearrangement of isoimide to polyimide.

N O

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